The subject matter disclosed herein relates to the fossil fuel burning gas turbine engines, and, more particularly, to the utilization of a secondary source of free or waste energy, in addition to the primary fossil fuel energy source, to increase the overall conversion efficiency of the system.
In a gas turbine engine air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases, which flow downstream to an expander, which extracts energy from the hot combustion gases. The temperature of the hot gas stream from the combustor of modern gas turbine engines is extremely high, typically well above 2500 degrees F. Such temperatures are comparable to or even higher than the melting point of the advanced alloys, which are used in the manufacture of turbine parts, e.g. nozzles or vanes (stationary), blades or buckets (rotating), and wheel spaces in between. These parts are commonly referred to as the hot gas path (HGP) components. Consider that, in the absence of cooling, the airfoils comprising the first stage of a modern gas turbine engine would melt away within a few seconds. Air used for cooling the HGP components in the gas turbine engine is typically extracted from the compressor discharge or inter-stage locations and is therefore not used in the combustion and turbine expansion process, and correspondingly decreases the overall efficiency of the gas turbine engine. The cooling air stream is referred to as “chargeable” airflow, the amount of which is controlled by the temperature of the pressurized air, which is channeled from the compressor to the turbine. In general, the further downstream the cooled turbine part in the expansion path, the further upstream is the coolant extraction point in the compressor. Mixing of the spent cooling air with the hot gas expanding through the turbine is a further source of lost work due to mixing and cooling losses.
In general, the energy input to the gas turbine via the heating value of the fuel burned in the combustor can be reduced by increasing the temperature of the fuel itself and/or the combustion air from the compressor discharge. The ensuing reduction in the amount of fuel burned in the combustor is reflected by the increase in gas turbine efficiency if a free/waste energy source is available to accomplish the said raise in the temperature of the fuel and/or combustion air. In modern industrial or heavy-duty gas turbines utilized in electric power generation, fuel gas performance heating (e.g. to 365 F or higher) using heat recovery boiler feed water is an established practice. Heating the compressor discharge air in a similar manner, while certainly possible in theory, is not feasible due to the very high temperature of the discharge air (e.g. nearly 800 F in advanced F-Class turbines) and the very closely integrated gas turbine structure.
Conventionally, e.g. in concentrated solar power (CSP) applications, free or waste energy is utilized for steam generation and power production in a steam turbine. For example, in the gas and steam turbine combined cycle (CC) power plant, the waste energy from the gas turbine exhaust is utilized to generate steam in a heat recovery steam generator (HRSG) for additional expansion and power generation in a steam turbine. Due to the relative position of the basic thermodynamic cycles representing the gas and steam turbines on a temperature-entropy surface, i.e. Brayton and Rankine cycles, respectively, the former is commonly referred to as the “topping” cycle and the latter as the “bottoming” cycle. In other systems, the steam generation in the HRSG of a CC power plant is supplemented by utilizing the (free) solar energy in a separate boiler section. In either case, the free or waste energy is utilized in the bottoming steam cycle of the CC power plant.